Heater for electron tubes



.lune 27V, 1967 H. G. SCHEIBLE 3,328,201

HEATER FOR ELECTRON TUBES Filed April 27. 1964 nmnanumwxmmmm 465 /V/'ii I0 I INVENTOR 5 Han/fea 65e/mue BY 0 WLM/m far/ley United States Patent O 3,328,201 HEATER FOR ELECTRON TUBES Howard G. Scheible, Livingston, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Apr. 27, 1964, Ser. No. 362,819 3 Claims. (Cl. 117-215) This invention relates to insulated heaters for electron discharge tubes, and particularly to thermally darkened insulated heaters.

One problem associated with the use of insulated heaters in electron discharge tubes is current leakage between the insulated heater and the cathode heated by the heater. As known, heater-cathode current leakage causes noise in the electrical output of the electron discharge tubes.

An object of this invention is to provide an improved heater for reducing heater-cathode current leakage in electron discharge tubes using the heaters.

It is known that the efliciency of heat transfer by radiation between an insulated heater and a cathodeheated by the heater is a function of the thermal color or thermal emissivity of the heater. The higher the eiciency of heat transfer, the lower need be the temperature of the heater to transfer a given amount of heat energy to the cathode. The lower the temperature of the heater, the greater is the life and reliability of the heater, as known.

Modern day electron tubes almost invariably use aluminum oxide as the insulation coating on the heater wire. Aluminum oxide has excellent insulating properties at high temperature, has a low vapor pressure, and does not react with either the heater wire or the cathode sleeve to affect the electrical or mechanical properties ofthe heater or cathode. Aluminum oxide, however, is white in color and has a low coeicient of thermal emissivity, Therefore, to heat a cathode to a given temperature by an aluminum oxide coated heater, it is necessary to operate the heater at a much higher temperature. For example, if the cathode temperature is to be 1100 C., the heater temperature may be 300 C. higher, or 1400 C.

It is known that adding tungsten powder to the aluminum oxide insulation raises its thermal emissivity. The tungsten powder darkens the aluminum oxide but does not adversely affect its other desired characteristics.

One problem encountered with the use of tungsten darkened heaters, however, is that due to careless preparation of the tungsten additive powder, or improper processing of the kelectron tubes containing the tungsten darkened heaters, the tungsten is sometimes oxidized to tungsten oxide. Although tungsten is non-reactive with aluminum oxide, tungsten oxide is reactive with aluminum oxide `at high temperatures and produces reaction products which allow current leakage between the cathode and heater through the insulation coating. Also, being volatile, the tungsten oxide vaporizes off the heater leaving the heater undarkened. In either event, the tubes containing the v tungsten oxide contaminated heaters are discarded.

It is a further object of this invention to provide an improved darkened -aluminum oxide insulated heater.

For achieving these and other objects, aluminum oxide insulated heaters are darkened by the vaddition of molybdenum disilicide to the aluminum oxide insulating coating. Preferably, the molybdenum disilicide is mixed with aluminum oxide and applied to the heater as a second coating on top of a first coating of pure aluminum oxide. Also, in Some instances, as described hereinafter, it is preferable to combine tungsten with the mixture of molybdenum disilicide and aluminum oxide. v

`In the drawings:

FIG. l shows an indirectly heated cathode in perspective and partly broken away to show within the cathode a 3,328,201 Patented June 27, 1967 coated insulated heater of a type which may be made according to this invention;

FIG. 2 is an enlarged view of a portion of the heater shown in FIG. 1;

FIG. 3 is a view similar to FIG. 2 but showing an alternative construction; and, l

FIG. 4 is a graph showing the variation of heatercathode current leakage wtih concentration of molybdenum disilicide in an outer heater coating for one electron tube type.

Many diverse types of cathodes and heaters are employed in electron tubes. FIG. 1 shows an example of one of such cathodes and heaters. The cathode 8 comprises a tubular hollow sleeve 10 having coated thereon an electron emissive material 11 capable of emitting electrons when it is heated to a suciently high temperature, Coating 11 may comprise a mixture of strontium and barium carbonates, or the like. The heater 14 shown for heating the cathode is known as a folded heater and comprises a plurality of folded strands of refractory wire 16, usually of tungsten, which are coated with an insulating material 18 to prevent electrical shorting between the various strands of the heater and between the base wire 16 of the heater and the cathode sleeve 10. Ends of the heater wire 16 extend outwardly from the cathode and are welded to electrical connectors 20 which provide terminals for the heater. To facilitate welding of the ends of the heater 14 to the connectors 20, the heater ends are uncoated.

The insulating `coating 1S for the heater 14 comprises a mixture of aluminum oxide and molybdenum disilicide. Molybdenum disilicide does not react with aluminum oxide, hence does not affect theinsulating characteristics of aluminum oxide. Also, the thermal emissivity of the aluminum oxide is markedly increased by the presence of the molybdenum disilicide.

A coating comprising a mixture of molybdenum disilicide and aluminum oxide may be applied as a second coating 22 covering a lirst coating 24 of aluminum oxide only which covers the heater wire 16, as shown in FIG. 2. Since the thermal emissivity of a body is primarily a surface phenomenon, heaters as shown in FIG. 2 have high thermal emissivities and may operate at temperatures considerably lower than undarkened aluminum oxide insulated heaters.

Using the molybdenum disilicide in the manner shown in FIG. 2, that is, by mixing it wit-h aluminum oxide and `applying it as a second coating on top of a rst coating of aluminum oxide only, has several advantages. Aluminum oxide is less conductive than molybdenum disilicide. Thus, by providing an undercoating 24y of aluminum oxide not mixed with molybdenum disilicide, the molybdenum disilicide in the second coating 22 is maintained out of contact with the heater wire 16 and does not create an electrical leakage path between the heater wire 16 and the cathode sleeve 10. Also, by mixing t-he molybdenum disilicide with the aluminum oxide in the second coating 22, the possibility of the formation of continuous paths of molybdenum disilicide through the aluminum oxide undercoating 24 4by the settling or diffusion of the molybdenum disilicide through pores or cracks in the aluminum oxide is minimized. Such continuous pat-hs might cause either electrical leakage between, or short circuiting of, the heater wi-re to the cathode sleeve. The use of molybdenum disilicide only in the upper or second coating 22 has the further advantage of reducing the amount of molybdenum disilicide required in the heater coating. Sin-ce molybdenum disilicide is a relatively expensive material, this reduces the cost of the heaters.

When heaters of the type shown in FIG. 2 are used in electron tubes, it is found that the electron tubes have longer life expectancies than electron tubes using undarkened heaters. It has been found, however, that with time molybdenum disilicide decomposes and causes changes in the electrical characteristics of the electron tubes using the heaters.

For obtaining longer life expectancies and minimizing the effects of the decomposition of the molybdenum disilicide, tungsten is combined with the molybdenum disilicide and aluminum oxide in the second coating 22', as shown in FIG. 3. By using both molybdenum disilicide and tungsten, both of which are darkening agents, smaller amounts of each material need be used than when either material alone is used to darken a heater.

An advantage of reducing the amount of tungsten in coating 22 is that during processing of the electron tubes, there is less likelihood that the tungsten will be oxidized. This is because the molybdenum disilicide and the -aluminum oxide `are mixed with the tungsten and tend to` protect the tungsten from oxidizing gases produced within electron tubes during processing thereof. Also, molybdenum disilicide is resistant to oxidation. Likewise, by reducing the amount of molybdenum disilicide in coating 22', the adverse effects of any decomposition of the molybdenum disilicide during the life of the electron tubes are lessened.

Tubes having aluminum oxide heaters darkened with molybdenum disilicide have lower current leakage between the heater and cathode than tubes having undarkened aluminum oxide coated heaters or aluminum oxide heaters darkened solely with tungten. Further, the greater the amount of molybdenum disilicide used in the heater coatings 22, 22', the lower is the cur-rent leakage. This is illustrated in the graph shown in FIG. 4, wherein the a'bcissa is percentage (percent) of molybdenum disilicide used in the second coating 22 of the heater shown in FIG. 3, by weight, as compared with the amount of tungsten used in the second coating, and the ordinate of `the graph is in micro-amps (I) of current leakage between the heater and cathode of the type RCA 6AF4 on which the data was taken. The heater to cathode measurements were made .after the test tubes were operated for 4,000` hours. As shown, below a concentration of around 110% (that is, 1 part of molybendum disilicide to 9 parts of tungsten in the coating 22'), the level of heater-cathode leakage rises abruptly.

The reason molybdenum disilicide reduces heater cathode current leakage is not fully known. It has been observed, however, that in tubes having molybdenum disilicide darkened heaters, a film of material is deposited 4on the inside wall of the cathode sleeve. Identiti-` cation of the material of the lm has not been possible, but it is believed to be either molybdenum disilicide `or a product of decomposition of molybdenum disilicide. Further, it appears that the presence of the lm prevents CII or inhibits diffusion of barium from the electron emissive coating on the outside of the cathode sleeve through the sleeve wall to the inside of the cathode. Inward diffusion of barium through the cathode sleeve tends to occur in tubes having tungsten darkened or undarkened heaters.

It is generally believed that the presence of barium on the inside of the cathode sleeve contributes to current leakage between the heater and cathode by electron emission. Therefore, the inhibition of the diffusion of barium by the presence of the cathode film is thought t0 reduce current leakage caused by electron emission. The reason for the ab-rupt rise in current leakage, as shown in IFIG. 4, is not known. In any event, however, the use of a sufficient .amount of molybdenum disilicide in insulated heaters results in electron tubes having exceptionally low heater-cathode current leakage.

lHeaters may be made with molybdenum disilicide darkened insulation coatings as follows:

For coating a wire as shown in FIG. 2, a 2 mil diameter tungsten heater wire may be rst coated in conventional manner with a coating 24 of aluminum oxide. The coating may be rapplied by drag coating. The coating suspension comprises:

1,0801 cc. of aluminum nitrate solution, comprising 2,000 cc. of distilled water and 2,984 grams of aluminum nitrate (Al(NO3)3-9H2O) per gallon;

650 cc. methanol;

390 cc. of distilled water; and

5,250 gm.-fused, .acid washed alumina, commercially available as Nortons grade 38-500 Alundum The 2 mil diameter heater wire is coated to a diameter of 7-8 mils by being passed through the above suspension 12 times` .at a rate of 1=4 meters per minute, the wire being air fired in an oven at 660 C. between passes.

The wire is then passed once at the same rate through a molybdenum disilicide and aluminum oxide suspension to provide about a 1/2 mil darkened layer on top of the rs-t layer of aluminum oxide. The molybendum disilicide suspension comprises:

4,300 gm. of the 4above-described aluminum oxide suspension used to provide the undercoating of aluminum oxide;

1,3 ml. partially hydrolized aluminum nitrate solution, commercially available as Sylvania B1 Binder; and

1,075 gm. molybdenum disilicide powder having an average particle size of 6 microns measured by the Fisher `Sub-Sieve Sizer. The molybdenum disilicide is drymilled for 4 hours prior to use to deagglomerate it.

2,083 cc. of the aluminum oxide undercoating suspension;

1,209 cc. of Sylvania Bl Binder;

545 gm. tungsten powder having an average particle size of 0.7 micron; and

545 gm. molybdenum disilicide powder having an average particle size of 6.0 microns.

VBoth the tungsten and molybdenum disilicide powders are balled milled for 4 hours prior to use to deagglomerate them.

The particle size of the molybdenum disilicide and the tungsten are not believed to be critical except that the smaller the particle size -of these materials, the greater is the darkening eifect. For tungsten, however, the smaller the `particle size, the more susceptible is the tungsten to being oxidized.

The preferred ratio of molybdenumdisilicide to tungsten in the coating 22 of the heater shown in FIG. 3 is 1 to 1 by weight. As shown in the graph of FIG. 4, this ratio (50% of molybdenum disilicide), provides the lowest current leakage of the concentrations tested. Higher concentrations of molybdenum disilicides are not recommended due to the tendency of the molybdenum disilicide to decompose after extended periods of time.

A satisfactory ratio of darkening material (molybdenum disilicide plus tungsten) the aluminum oxide in the coating 22' of the heater shown in FIG. 3 is 1 to 3 by weight. This provides adequate darkening of the heater while not providing excessive amounts of tungsten and molybdenum disilicide. This ratio is provided in the examples given.

After the heater wire is coated, the coated wire mayV be formed into heater structures of the type shown in FIG. 1 in any known manner.

What is claimed is:

1. A heater for a cathode of an electron tube comprising a wire structure having a coating thereon, said coating comprising a rst coating of aluminum oxide in Contact with said Wire structure, and a second coating of a material in contact with said iirst coating, said :second mixture comprising a mixture of aluminum oxide and molybdenum disilicide.

2. A heater for a cathode of an electron tube comprising a wire structure having a coating thereon, said coating compri-sing a rst coating of aluminum oxide in contact with said Wi-re structure, and a second coating of a material in contact With `said rst coating, said second coating comprising a mixture of aluminum oxide, molybdenum disilicide, and tungsten.

3. A heater for a cathode of an electron tube comprising a Wire structure having a coating thereon, said coating comprising a first coating `of aluminum oxide in contact with said wire structure, and a second coating of a ma- References Cited UNITED STATES PATENTS 2,745,928 5/1956 Glaser 219-553 X 2,745,932 5/1956 Glaser 117-69 X 3,195,004 7/1965 Hassett 117-217 X FOREIGN PATENTS 731,616 6/ 1955 Great Britain.

ALFRED L. LEAVIIT, Primary Examiner. C. WEIFFENBACH, Assistant Examiner. 

1. A HEATER FOR A CATHODE OF AN ELECTRON TUBE COMPRISING A WIRE SRUCTURE HAVING A COATING THEREON, SAID COATING COMPRISING A FIRST COATING OF ALUMINUM OXIDE IN CONTACT WITH SAID WIRE STRUCTURE, AND A SECOND COATING OF A MATERIAL IN CONTACT WITH SAID FIRST COATING, SAID SECOND MIXTURE COMPRISING A MIXTURE OF ALUMMINUM OXIDE AND MOLYBDENUM DISILICIDE. 